Please, Some Ancient Mixolydian Choral Music!

Guys, please, I've never found it would be so difficult...

I'm looking for some choral pieces written in strict mixolydian mode, but I don't want a modern one. I want those old medieval pieces, or maybe from renaissance. It just has to be mixolydian and choral. (preferrably for not too many voices...)

Thanks in advance^^

A little googling turned up the Bach 'Fughetta on Dies sind die heil'gen zehn Gebot' if you get desperate and want to throw in some organ.  Apparently Orlando di Lasso's "Stabat Mater" for eight voices from his "Five Latin Hymns" is in Mixolydian.  You could see if someone has put that up on choralwiki.org yet.  Someone seems to be making a project of getting all his works up there, since they are in the public domain.  More and more of his compositions have been turning up over the last year of so.  (:

Also found a reference to the "Catigas de Santa Maria" 400 songs about Mary assembled under the direction of King Alfonso el Sabio between 1250 and 1280, most "in dorian or mixolydian modes."  Someone may have put them up online as well, since they would also be public domain.  

I'd also add that this may be a job for a research librarian.  They are trained in the fine art of finding out information.  If there is a music library you can access, so much the better, but just the research desk at your regular library might have a wonderful time hunting down some ideas for you.  Librarians live for questions like this.  

Thank you all for your suggestions. I found some of these pieces myself before, so I'll be more specific to what I want:

I would like to find some easy-to-sing, one to two parts vocal music in mixolydian mode, as I want to use it with my students at the school where I'm teaching music. As they'll sing it, I can't use an 8-part mass (even though it's beautiful).

So, maybe gregorian chants are the ones I'm looking for...

Thanks again^^

Note that due to use of musicae ficta, the difference with Ionian will be very subtle.